1. Field of the Invention
The invention concerns electrode configurations for forming field cages, used especially in Microsystems for handling and manipulating particles, and field cage electrodes for use in such electrode configurations.
2. Description of the Related Art
To produce certain states or for the operation of certain processes in numerous biotechnical, medical, gene engineering and chemical engineering systems, it is necessary to hold microscopic particles precisely in free suspensions or to move them in a predetermined manner. The particles of interest include biological cells or parts of them, latex particles or other socalled microbeads. By the influence of electrical fields in socalled field cages the particles are moved (xe2x80x9copenxe2x80x9d field cages) or held (xe2x80x9cclosedxe2x80x9d field cages) in the suspension. In a held state the particles can be measured, for example, or caused to interact with one another (cf U. Zimmermann et al. in xe2x80x9cElectromanipulation of Cellsxe2x80x9d in CRC, 1996, chapter 5, pp 259-328).
The fields for influencing the particles are formed of electrodes, preferably fabricated by semiconductor technology methods. To form a field cage, a plurality of electrodes are configured either two- or three-dimensionally and potentials are applied to them so that, in the space enclosed by the electrodes (the socalled inner space), a field distribution is created in which a particle can be trapped or moved in a particular direction.
The electrode shape and configurations for microsystem applications were only optimized to date in terms of the effectiveness of cage formation in the inner space (cf S. Fiedler et al. in xe2x80x9cMicrosystem Technologyxe2x80x9d 2, 1-7, 1995). It is known, for example, that electrodes can be formed with a strip form tapering hyperbolically into the inner space for field cages of several micrometers to several hundred micrometers. Such simple electrodes were regarded to date as optimal in terms of field cage formation and their optimization through relatively simple computation of the field gradients. Nevertheless, the following effect is a disadvantage in conventional electrode configurations and shapes.
If the particle density in a liquid (suspension) is low, single particles, eg a cell, can be held stably in the field cage for a long period (minutes to hours). At higher particle densities however, further particles enter the field cage in the course of time, which is generally undesirable. This effect is initially surprising, because the field cage represents a potential barrier intended to prevent migration of particles into the inner space. But there are extra forces present, directed towards the inner space, that allow the particles to overcome the potential. These forces are the result of thermal convection, especially warmup near the electrodes.
As a result of this effect, the use of field cages in microsystems is restricted to relatively small particle densities, which are frequently unacceptable in practical terms.
The object of the invention is to present an improved electrode configuration for cage formation and improved field cage electrodes for such an electrode configuration, with which larger particle concentrations can be processed, with greater stability and reliability, than with conventional Microsystems.
This object is solved by an electrode configuration and a field cage electrode of the present invention.
The invention is based on the idea of creating new electrode forms and configurations that, on the one hand, reduce or suppress thermal convection flows and, on the other hand, enhance the effectiveness of the potential barrier surrounding the field cage. Electrode configurations according to the invention consist of a large number of field cage electrodes, each with an end or head region to which electrical potential can be applied through a feed region. Unlike conventional electrode forms with smooth, uniform edges, the end regions of the electrodes in the invention are shaped so that highly inhomogeneous fields are created in both the inner space (field cage) and the outer space. Here the inhomogeneity of the fields is so strong, or the field gradients so pronounced, that a shielding (or screening) field forms outside the field cage. The shielding field should be strong enough for forces to act on the particles that compensate for the forces directed inwards (eg through thermal convection). For this purpose the end region of an electrode exhibits electrode segments that, in part at least, are limited by straight or slightly curved edges that abut against one another at predetermined angles. The edges abut against one another in such a way that an discontinous demarcation (formation of a corner or tip or the like) results. This means that high field line concentrations form on the edges of the end region, producing the required, large field gradients.
In a preferred embodiment of the invention, in addition to the above shaping of the end region to create inhomogeneous shielding fields, the feed region, through which the end region is connected to an electrode terminal, has an electrode surface as small as possible. Preferably the feed region is of strip form with a width optimized for the electrical power. The formation of feed regions that are as narrow as possible leads to a reduction of thermal convection.
The invention, unlike the electrode forms used to date and optimized for field cage creation, produces instead electrodes whose end regions allow the formation of field line concentrations, eg on edges or tips of the electrodes.
Electrode configurations according to the invention can be arranged planar two-dimensionally, the holding or moving of particles then being produced by interaction of the field cage with parts of the microsystem (mechanical limiting). Alternatively the electrode configurations may take on a three-dimensional form in which the particles are only held in the field cages by the effect of the electrical forces. But in the case of three-dimensional systems too, mechanical limiting can cooperate with the field cages.
In a special embodiment of the invention, not only the end regions of the electrodes but also the feed regions are provided with electrode segments that enable the formation of strong field gradients. With suitably designed interaction of adjacent field cage electrodes in particular, this allows open and closed field cages to be combined, eg in the form of a field cage with a feed channel.
Methods and devices according to the invention can be used in correlation spectroscopy, especially for verifying fluorescent molecules on the surface of submicrometer or micrometer particles and/or cells, or in pharmacological, medical diagnostic and/or evolutionary biotechnical applications. In particular, fluorescence correlation spectroscopy (WO 94/16313) and other, especially confocal fluorescence techniques, as proposed in WO 96/13744 and European patent application 96116373.0, can be used as verification methods. The last mentioned application suggests a method for analyzing samples by repeated measurement of the number of photons per predetermined time interval of electromagnetic radiation, especially light, emitted, scattered and/or reflected by particles in the sample, and determination of the distribution of the number of photons in the particular time intervals, whereby the distribution of the molecular intensities of the particles is found from the distribution of the number of photons.